Process for the production of a highly disperse pigment



- presence of about solution of any desired COIlcGfltl'SfiOIl,

pH value ofwhichis kept constantly between 2' and 6 is subjected to 1content-of at least about 50%, and a 7 During the electrolysis sludgePatented June 19, 1945 rnoonss Foa HIGHLY DISPERSE THE PRODUCTION OF APIGMENT Wilhelm Miller and Leonhard Spies, Bitterield, Germany; vestedin the Alien Property Custodian I No Drawing, Application May 11, 1939,Serial No. 273,046. In Germany May 16, 1938 I 6 c aims. c1. 2s-.-5s)

This invention relates to a process for the production of ahighlydisperse pigment.

. According to a known proposal a diflicultly albeit positivelysoluble,. highly disperse pigment having passivating properties isobtained by subjecting readily water-soluble, complex potassiumironchromate to a heat treatment in an autoclave at a temperature of about200 C. in the twice the amount by weight of water. .When used as acoating medium, this complex chromate exhibits the property of causing acertain amount of CrO4 ions to dissolve on contact with water, whichions then effect a pronounced and comparatively persistent passivationand thereby increase the corrosion resistance of the surface of thecoated metal. This proposal is, however, somewhat cumbersome since thewater-soluble potassium-iron chromate must first be made and thensubjected to the autoclave treatment at elevated temperature, whichtreatment is dimcult to control.--

The present invention aims at providing a simthe production of complexp- To this end, according to the invention, an electrolytewhich'consists of a potassium salt, e. g. a'potassium chromate or p tasium chloride and the electrolysis'using an yanode of a ferrochrome alloyhaving an initial chromium any desired electrolyte metal. To keep thepHvalue of the within the range hereinbefore set forth, a caustic potashsolution is preferably added to the electrolyte during theprocess with[constant stirring, thereby neutralising the acid formed during theelectrolysis.

is formed .at the anode and is-deposited at the bottom of the vessel.

The sludge which consists .of potassium-iron chromate of the approximatecomposition:

v 11K: O.8Fea0a.3Ci'iOa.28CrO:.18H-z0 is removed from .the cell fromtime cathode of .plate of? I m gma and is thereupon washed out withwater. e washing water may be used, if desired. for making up theelectrolyte solution, the volume ofwhich has been reduced either bypartial evaporation or by being entrainedwith the anode sludge. I

The dried, ochre-brown potassium-iron chromate is found'to be a pigmentof excellent quality. About by weight of the unground product is of agrain size of under 5 "electrolyte which During electrolysis which isconducted lyte from container at that the proportion of The highdispersive ness of the potassium-iron chromate produced by thehereindescribed process imparts a high covering power to the paints madetherewith.

A further advantage arising in the use of potassium-iron chromate,produced in accordance with the invention, metals which are exposed tocorroding agents, has been found to be that it forms-a passive layerlargely preventing thecorrosion oi the surface of the metal inconsequence'oi its property of causing small amounts of chromate ionsto. dissolve in the presence of water. 7

The potassium-iron chromate pigment of the present invention may of itsgood passivating properties, with other colour pigments which do notpossess erties. I

The potassium-iron chromate pigment of the present invention may beproduced. in the following manner. 1

The potassium-iron chromateipigment of the present invention may, beproduced in the following manner: 1

ferrochrome alloy containing 66.4% of chromium is immersedas contains200 grams of KCl per value of which has been adhydrochloric acid.

voltage 'of4 volts and an anodic current density of 0.,1 amp. per squarecentimetre, a certain quantity of caustic potash lye is added to theelectrotime to time in-order to maintain the pH value thereof at orabout its Electrolysis results in sludge consisting of the desiredpotassium iron chromate, which is removed from the electrolyticintervals of about 4 days each. The removed product is washed severaltimes with hot water while being stirred and thereupon dried at C. Apulverulent ochre-brown product consisting of potassium iron chromate isthusobtained, 80% of the grains of which are of asize of less than 5;. II

If desired, ,the potassium-iron chromate pigmerit produced inaccordancewith the invention can also be subjected to a subsequentpressure treatment at about 200 C. by the-known method hereinbeforereferred to. In this case, the compigme'r'it is changed in the sense K20in the compound is lowered while the proportions of FezO3 and CH0:

C10: is also inpositionof the as a coating medium for also be mixed, onaccount these prop- An ironv sheet is immersed as cathode, and .a

anode in an aqueous original level. the formation of an anodic If thisproduct is treated in a closedvessel for two hours at 200 C.

with addition of twice the amount by weight of water, the solidconsistingessentially of iron chromium with at least about 50% ofchromium, while permanently maintaining a pH value of the elecbetweenabout 2 and about 6.

with at least about 50% of chromium, while permanently maintaining a pHvalue of the electrolyte of between about 2and about 6.

3. A process for the with at least about 50% about of chromium, whilepermanently maintaining a pH' value of the electrolyte of between 2 andabout 6, removing the potassium iron chromatesludge formed from theelectrolyte and drying the same.

5. A process for the production of a highly disperse potassium ironchromate pigment, which Of iron chromium of chromium, while permanentlymaintaining a pH value of the electrolyte of between 2 and about 6,removing the potassium iron chromate sludge formed from the electrolyte,suspending said sludge in about twice the amount by weight of water andtreating the suspension-in a closed vessel at about 200 C.

WILHELM Mil FILER. LEONHARD SPIES.

